This invention relates to an autotensioner for keeping the tension in a belt of a belt transmission at a constant level.
An automotive internal combustion engine has a belt transmission. While the engine is running, the tension in the belt of the belt transmission tends to fluctuate rather violently due to changes in distance between pulleys resulting from thermal expansion of the engine body, or variations in torque. To prevent such variations in belt tension, i.e. to keep the tension in the belt at a constant level, a hydraulic autotensioner is used.
FIG. 4 shows a conventional autotensioner of this type. The autotensioner shown comprises a cylinder 40 filled with hydraulic oil and having its top opening closed by a seal member 41, a rod 42 slidably extending through the seal member 41, a rod spring 43 biasing the rod 42 outwardly of the cylinder 40, and a hydraulic damper 44 provided at the bottom end of the rod 42.
The hydraulic damper 44 includes a plunger 45 mounted in the cylinder 40 so as to be slidable along its inner peripheral surface. The plunger 45 defines a pressure chamber 46 and a reservoir chamber 47 in the cylinder 40. The chambers 46 and 47 communicate with each other through a passage 48 formed in the plunger 45. The damper 44 further includes a check valve 49 for opening and closing the passage 48.
Biased by the rod spring 43, the rod 42 is pressed against a roller arm 33 pivotable about a pin 32, so that its tension roller 34 is pressed against a belt A, thus tensioning the belt.
While the engine is running, the belt A vibrates due to fluctuating torque. The rod 42 is thus intermittently pushed in by the belt A. When the rod 42 is pushed in, the pressure in the pressure chamber 46 exceeds the pressure in the reservoir chamber 47, and the passage 48 is closed by the check valve 49. The pushing force acting on the rod 42 is thus damped by the hydraulic oil sealed in the pressure chamber 46.
While the pushing force acting on the rod 42 is greater than the force of the rod spring 43, hydraulic oil in the pressure chamber 46 is allowed to leak through a gap present between the outer surface of the plunger 45 and the inner surface of the cylinder 40 into the reservoir chamber 47, so that the rod 42 and the plunger 46 will lower slowly until the pushing force on the rod 42 balances with the force of the spring 43. This prevents excessive tensioning of the belt.
When the belt A slackens, the rod 42 is pushed out by the rod spring 43. As soon as the rod 42 begins to move outward, the pressure in the pressure chamber 46 drops below the pressure in the reservoir chamber 47, so that the check valve 49 opens the passage 48, allowing the rod 42 to move quickly outwardly following the slackening of the belt A. The slackened belt is thus quickly tensioned again.
Even after the engine has been cut, the belt may remain tensioned according to the positions of the cams on camshafts.
If the belt tension remains high after the engine has been cut, the rod 42 is urged into the cylinder 40 by the belt, so that hydraulic oil in the pressure chamber 46 leaks through the gap between the plunger 45 and the inner periphery of the cylinder 40 into the reservoir chamber 47 until the plunger 45 sinks rather deep into the cylinder.
When the engine is restarted in this state, the cams move and the belt will slacken quickly. Thus, the rod 42, which has been pushed deep into the cylinder, has to move a long distance until it is pressed against the belt. While the rod is protruding chasing the belt, the belt is free to vibrate. It thus takes a long time to stabilize the belt immediately after the engine has been started.
An object of this invention is to provide an autotensioner having a means for preventing its rod from being pushed in if the belt is tensioned according to the positions of the cams on camshafts when the engine is cut, thereby keeping the belt from slackening when the engine is restarted, namely, stabilizing the movement of the belt at the start of the engine.